It is well known that dry cement must be prepared in equipment specially designed for efficient and complete mixing, mostly with other materials such as sand and gravel. The inevitable result of mixing cement in one location and thereafter removing the mixed cement to pour it at a remote location is that all surfaces of the mixing equipment, transport equipment (such as cement trucks), concrete pumps, and similar equipment that come into contact with the mixed cement must be washed free of the substantial amounts that adhere to those surfaces.
It has been known for hundreds of years that cement-containing slurries behave thixotropically. Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick (viscous) or essentially solids under static conditions will flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed (time dependent viscosity). They then take a fixed time to return to a more viscous state. Some thixotropic fluids return to a gel or solid state almost instantly, such as cement containing slurries, and are called pseudoplastic fluids. Flow can induce reversible and irreversible structural changes in dispersions. The analysis of flow effects on microstructure and rheology remains one of the challenging problems in colloid science. The rheological manifestation of flow-induced structural changes is variable viscosity. If the changes are reversible and time dependent, the effect is called thixotropy. The microstructural changes due to flow are quite complex and not fully understood.
Compounding the challenges in this complex field is the problem presented by thixotropic, water-containing slurries and semi-solids from washout of cement processing and handling equipment. These washout slurries necessarily contain widely variable proportions of cement, water, gravel (aggregate), polymer enhancers, dirt, construction site waste, and other materials. Once the washout materials make it into a large dump truck or other very large watertight container for transport away from a construction site, the problems presented by their thixotropic states is essentially eliminated, in that the extremely large containers aggregate such a huge mass of the materials that inclining the large container results in fairly easy sliding of the static mass out of the large container.
The problem of forming and then needed to remove essentially a solid state thixotropic material in a small volume, watertight container, i.e., a construction site washout pan has existed for decades. The prior art has stagnated in the field of concrete washout pans. A highly visited website illustrating the fixation of those skilled in the art to a few designs is found at the website www.washoutpan.com, operated by Washoutpan.com, which also shows those designs in its catalog “Brigade Pro Washout Pans”. The prior art is dominated with rectangular shaped open pans, often with slanted side walls or with variable depth bottom surfaces. These are all formed of heavy gauge welded steel and fabricated to meet environmental protection regulations requiring that the pans be watertight so that washout materials do not fall onto open ground.
The relatively shallow depth and small capacity of washout pans is a result of the high density of wet concrete at about 2400 kg/m3 and structural limitations of the pans and lifting equipment such as cranes and forklifts. Washout pans can be easily placed in a convenient location for washout of cement handling equipment. However, the washout pans, once filled, must be lifted horizontally by crane or forklift without spilling any of the contents to the large transport dump truck or trailer sized container. Once the washout pan has been positioned over the large container, it is lifted into a vertical position so that the settled, nearly solid, thixotropic materials can be encouraged to fall out. This gravity-only encouragement is generally insufficient to cause the settlement, sticky, thixotropic materials from falling out of the washout pan into the large container. The workers responsible for emptying the washout pans regularly must pound on the walls and floor of the washout pan to induce sufficient agitation at the interface between the thixotropic, cement-containing slurry and the metal surface of the washout pan to reduce the highly viscous bond that has formed quickly between them in the short time the materials have been in the washout pan. Transportation of the washout pan, while inducing vibration into the thixotropic materials, actually makes the problem of getting the material out later even worse—the more viscous and sticky materials settle to the bottom of the washout pan in transportation, forming an intimate bond between the cement-containing materials and the floors and walls of the washout pan.
Washout pans generally have an effective capacity of from 150 to 250 gallons and the washout pan must be capable of holding a watertight load during transport to a larger container. The total weight of these cement-containing materials is regularly over 5,000 pounds. The repeated filling, lifting of an extremely heavy, shifting weight in the pan, and dumping of the washout material into a larger container for transport to a processing facility results in metal fatigue and bending. Even a small leak in the washout pan can bring stiff fines for environmental contamination of a building site. When responsible workers regularly have to pound heavily on the pan walls and floors with hammers to dislodge the materials when the pan is in a vertical position, the life of the washout pan becomes extremely short.
There is a need for a washout pan that virtually eliminates the need for worker-induced vibration to release cement-containing, thixotropic materials when the pan is moved into a vertical position, where stress at the materials-pan surface interface is automatically induced to release the cement-containing materials by the nature of the construction of the washout pan and its response to being moved from a horizontal to a vertical position.